Typically, the most common electrical energy storage devices are electrochemical batteries and capacitors, including supercapacitors. The device disclosed herein is an implementation of a hybrid lead acid battery and activated carbon supercapacitor, having features and performance characteristics that are distinct from either a battery or a supercapacitor.
The energy in this type of hybrid device is stored both electrostatically and electrochemically. The device in keeping with the present invention has a significantly greater cycle life and shorter charge time than a lead-acid battery. The present device also has a much higher energy density than a conventional supercapacitor. Like a conventional supercapacitor, the present device has a linear voltage profile when cycled at constant current. While this type of device typically requires a power conversion interface for many applications, it also delivers an accurate instantaneous mapping of its state of charge. Since the lead electrodes of the energy storage device disclosed herein are very similar to conventional lead-acid battery electrodes, many common components can be used, as well as many common strategies, methods and designs for tuning and enhancing performance.
One problem with the use of conventional lead-acid battery components within devices such as those disclosed herein is that the current collection methods needed for the carbon electrodes are significantly different than those of the lead electrodes. For instance, because of the lesser conductivity of carbon electrodes, the need for maximum surface contact and a short electrical path between the carbon electrode and the underlying collector assembly is paramount. Another problem is corrosion of the current collector. A further problem is the contact resistance between the current collector and the carbon electrode.
In these respects, the disclosed cell assembly for an energy storage device, according to the present invention, substantially departs from the conventional concepts and designs of the prior art, and in so doing provides a flexible and economical way to create a multi-plate, multi-cell, hybrid lead acid battery/supercapacitor energy storage device.
A significant factor in assembly of energy storage devices such as those disclosed herein is that it is desirable that the electrodes be essentially self-contained and free standing, so that they can be easily handled during automated assembly operations. That is achieved in the present invention by assuring that the activated carbon in the negative electrodes is not only in electrical contact with a conductive shield of current collector assembly, but that the activated carbon is also adhered to the conductive shield material. This ensures against settling or shedding of the carbon material, and thereby loss of electrical contact, and, therefore, loss of capacity in the electrode.
Moreover, the necessity for excessive compression of the assembled electrodes in a manufactured, assembled energy storage device of the present invention is obviated. The electrodes need only to be compressed to the same extent as electrodes of an ordinary lead-acid battery. Thus, assembly of energy storage devices in keeping with the present invention may be effected on manufacturing assembly lines that will resemble—and to quite an extent emulate—those of a lead-acid battery manufacturing assembly line. However, the specifics of any such manufacturing assembly line as might be employed to manufacture and assemble energy storage devices in keeping with the present invention are beyond the scope of the present disclosure.
It will be noted that the assignee of the present invention is also the assignee of an invention relating to hybrid energy storage devices which require significant compression of the electrodes when the devices are assembled. The referenced application was originally filed as provisional Specification Serial Number 60/730,397 filed Oct. 27, 2005.